The polymerization of lower .alpha.-olefins to produce polyolefins is an industry of substantial significance. In a conventional polymerization process, regardless of process type, the polymer product is removed from the polymerization reactor periodically or continuously and is separated from the other product mixture components. The polyolefin is typically recovered as granules whose size and shape depend in part upon the particular catalyst and process used. As a part of converting the initial polymerization product to a marketable commodity, it is usually necessary to provide one or more additives to the polymer product to improve stability or other characteristics which may be determined by the particular end use contemplated for the polymer. The applicable additives include thermo-oxidative stabilizers, ultraviolet light stabilizers, processing stabilizers, acid acceptors and other materials chosen to modify the processability and properties of the polymeric substrate. The provision of such additives to the polymer does present some difficulty on occasion.
One conventional method of incorporating additives such as stabilizers into a polyolefin polymer involves the extrusion step typically employed to produce the polymer in pellet form. Stabilizers are mixed with the polymer feed to the extruder and stabilized compositions are produced by the mixing associated with the extrusion. The resulting stabilized polymer is in the physical form of pellets and much of the commercial polyolefin, particularly polypropylene, is marketed in this form. The extrusion method, although widely practiced, has disadvantages in that the polymer and/or additive may degrade under the conditions of heat and shear encountered in the extrusion. Further, the extrusion process is energy-intensive.
In U.S. Pat. No. 3,591,409 there is disclosed a method of coating resin particles which involves high intensity mechanical mixing of a polymer with additives and a binder. In U.S. Pat. No. 4,517,246 there is claimed a polymeric material which has been coated with additives and a binder through the use of a mechanical mixer. In U.S. Pat. No. 4,960,617, Chatterjee, there is disclosed a method of stabilizing polypropylene after passage from the polymerization reactor. This method involves application of additives dispersed in hot, molten wax. In U.S. Pat. No. 3,271,185 a process is described wherein vaporizable antioxidant is added to a fluidized bed of polymer. In Chatterjee, U.S. Pat. No. 5,141,772 a process is described for applying additives in the form of an additive aqueous dispersion to a fluidized bed of polymer granules. Further, U.S. Pat. No. 4,517,246 discloses the use of a batch-type mixer to produce a polyolefin molding material coated with organic binder and additive such as filler.
It would be advantageous to provide an improved method of applying additives to polymeric materials, as well as the additive-incorporating polymer compositions produced by that process.